Clubroot is a widespread disease that causes serious problems in many Brassica growing areas. See, e.g., Dixon (1999) Grower April 29:28-9. The disease is caused by Plasmodiophora brassicae, a unicellular organism. Symptoms of the disease include root malformations with hard swellings (clubs) that eventually rot. The disease also causes stunting through reduced growth, and wilting of leaves is observed under water stress. Chemical control of the disease is not effective.
The genus of the Brassica comprises several species of commercial interest, such as B. rapa (e.g., Chinese cabbage, pak choi, turnip); B. napus (e.g., oil seed, swede); B. juncea (e.g., mustard); B. nigra (e.g., black mustard); and B. oleracea (e.g., cauliflower, broccoli, cabbage, Brussels sprouts, savoy cabbage, borecole, kohl rabi, borecole, etc.). While subspecies within a species of the Brassica genus are usually sexually compatible, this is not necessarily the case between different species of the Brassica genus. For example, B. rapa and B. oleracea do not have the same number of chromosomes (10 chromosomes versus 9 chromosomes), and are therefore not sexually compatible. This renders the transfer of a trait from one Brassica species to another particularly difficult.
Several sources of resistance to clubroot have been described within the Brassica genus. See, e.g., Bradshaw et al. (1997) Ann. Appl. Biol. 130:337-48; Gowers (1982) Euphytica 31:971-6. Some resistances are monogenic, some polygenic, some are dominant, and some are recessive. Monogenic dominant resistances have been described in B. rapa and B. napus, such as, for example, a monogenic dominant resistance in the B. rapa Chinese cabbage. Yoshikawa (1983) Japan Agricultural Research Quarterly 17(1):6-11. Chinese cabbage F1-hybrids with this resistance have been shown to have good protection against clubroot, although a small number of strains (“races”) of clubroot have been able to break through this resistance.
Clubroot disease infection is considered to be a major threat to the canola cultivation industry. Attempts to breed for clubroot resistant canola varieties will produce significant number of segregating plants that need to be screened for clubroot disease resistance. The current practice of screening breeding materials for clubroot disease resistance includes phenotyping individual plants through observation of that plant's phenotypic reaction after inoculation of the plant with the P. brassicae pathogen. This already cumbersome and time-consuming process is further complicated by current regulatory restrictions on the transport of clubroot pathogen or any infected materials. The foregoing considerations make the cost of screening breeding materials for clubroot disease resistance extremely high, and also significantly hinder breeding efforts by limiting the number of lines that may be tested at one time.